Stenosis of a blood vessel causes narrowing of the vessel at the location of the stenosis. This narrowing effects blood flow and, if further blockage occurs, can cause damage to tissues supplied by the vessel. For example, when stenosis occurs in a coronary artery, the blood supply to the portion of the heart supplied by that artery may be compromised. If the stenosis is severe, there is an increased risk of myocardial infarction.
Various methods are known to measure the degree of obstruction caused by a stenotic lesion in a vessel. Some methods rely on visual observation during the injection of contrast media. A more precise evaluation can be made by directly or indirectly measuring the flow of blood across the lesion. Such measurements may then be used to determine whether or not the degree of stenosis is sufficiently severe that intervention is warranted, and what that intervention should be.
One measurement of the severity of stenosis in a blood vessel is the Fractional Flow Reserve, or FFR, which is calculated based on pressure measurements. To calculate the FFR for a given stenosis, two blood pressure readings are taken. One pressure reading is taken on the distal side of the stenosis (e.g., downstream from the stenosis), the other pressure reading is taken on the proximal side of the stenosis (e.g., upstream from the stenosis and closer to the aorta). The FFR is defined as the ratio of maximal blood flow in a stenotic artery, taken distal to the lesion, to normal maximal flow, and is typically calculated based on a measured pressure gradient of the distal pressure to the proximal pressure. The FFR is therefore a unitless ratio of the distal and proximal pressures. The pressure gradient, or pressure drop, across a stenotic lesion is an indicator of the severity of the stenosis, and the FFR is a useful tool in assessing the pressure drop. The more restrictive the stenosis is, the greater the pressure drop, and the lower the resulting FFR.
The FFR measurement may be a useful diagnostic and treatment planning tool. For example, clinical studies have shown that an FFR of less than about 0.75 may be a useful criterion on which to base certain therapy decisions. An example of such a study is Pijls, DeBruyne et al., Measurement of Fractional Flow Reserve to Assess the Functional Severity of Coronary-Artery Stenoses, 334:1703-1708, New England Journal of Medicine, Jun. 27, 1996. A physician might decide, for example, to perform an interventional procedure (e.g., angioplasty or stent placement) when the FFR for a given stenotic lesion is below 0.75, and may decide to forego such treatment for lesions where the FFR is above 0.75. In other studies, the cut off value for the FFR at which an intervention is performed is 0.80. Thus, the FFR measurement can provide a decision point for guiding treatment decisions.
One method of measuring blood pressure for use in calculating FFR is to use a pressure sensing guidewire. Such a device has a pressure sensor embedded within the guidewire itself. A pressure sensing guidewire could be used in the deployment of interventional devices such as angioplasty balloons or stents. To use a pressure sensing guidewire, in certain applications the guidewire must be repositioned so the sensing element of the guidewire is correctly placed with respect to a stenotic lesion, for example. Blood pressure measurements for calculating FFR, for example, are generally taken on both sides of a given stenosis, and one way in which the upstream measurement could be made would be to retract the guidewire across the stenosis to make the upstream measurement. The guidewire may also be retracted across the stenosis in order to normalize the pressure sensor to an aortic pressure. After retracting the guidewire to make the proximal pressure measurement or to normalize the pressure, the guidewire may again be repositioned downstream of the lesion, for example, if it is determined (e.g., based on the FFR calculation) that an interventional device should be deployed. In cases where there are multiple lesions, if the guidewire is used to make a proximal pressure measurement, the sensing element of a pressure sensing guidewire would need to be advanced and retracted across multiple lesions, and would potentially have to be advanced and repositioned again for each such lesion. Advancing and maneuvering a pressure sensing guidewire though stenotic lesions and the vasculature, for example, can be a difficult and/or time consuming task.
Physician preference is another factor that may influence the choice of diagnostic tools or techniques used for certain applications. For example, some physicians may tend to become accustomed to using certain specific guidewires for certain applications. “Standard” (e.g., commercially available) medical guidewires may vary in size, flexibility, and torque characteristics. A physician may prefer to use different guidewires for different tasks, for example, to access hard-to-reach anatomical areas, or when encountering bifurcations in arteries. Certain guidewires may therefore be better suited for specific tasks because of the torque and flexing characteristics, and a physician may display a strong preference for using a certain guidewire based on the specific task (or tasks) he or she is facing. A pressure sensing guidewire may have torque and flexing characteristics that are either unknown to the physician, or that are unsuitable for a particular task, because such a guidewire is specifically constructed to have a pressure sensor incorporated as part of the guidewire itself. As a result, a physician may find it difficult to maneuver a pressure sensing guidewire into an anatomical location of interest, as compared to a “standard” (e.g., non-pressure sensing) medical guidewire.
Having grown accustomed to the handling characteristics of a particular standard, non-pressure sensing guidewire, a physician may be reluctant to employ a pressure sensing guidewire, which may increase the time and difficulty of positioning and repositioning the pressure sensing guidewire across a stenotic lesion, for example. In such cases, a physician may choose to forego the benefit of a diagnostic measurement, such as FFR, and simply choose to deploy some form of interventional therapy as a conservative approach to such decisions. If the diagnostic measurement techniques and the associated devices were simple enough to use, more physicians would use them and thereby make better therapy decisions.
It should also be noted that when pressure measurements are made using a pressure sensing guidewire, some error may be introduced due to the cross sectional size of the guidewire, which typically has an outer diameter of about 0.014 inches. This is because, as the guidewire crosses the lesion, the guidewire itself introduces blockage, in addition to that caused by the lesion itself.
The measured distal pressure is therefore somewhat lower than it would be without the additional flow obstruction caused by the guidewire. The presence of the guidewire within the artery may therefore exaggerate the measured pressure gradient across the lesion. Nevertheless, many clinical studies evaluating FFR, which are used to determine the FFR at which various interventions should be employed, use pressure sensing guidewires to measure and calculate the FFR cutoff. As such, the values determined in these clinical studies are offset from the true FFR by the amount of the error caused by the presence of the pressure sensing guidewire. However, measurements obtained using this method do not necessarily need to be corrected for this error, since the values against which they are compared for making treatment decisions (the values based on clinical studies) also include this error if they are obtained in the same way (that is, using a pressure sensing guidewire). Therefore treatment decisions may be made using the FFR obtained using a pressure sensing guidewire without correcting for the error caused by the presence of the guidewire.